Optical isolators are key elements required in most free space optic and fiber optic applications. The most commonly used type of isolator is based around a Faraday rotator crystal. FIG. 1 is a schematic diagram that illustrates a typical configuration of a conventional optical isolator 5. An input polarizer 10 of isolator 5 is typically aligned to a linear polarization angle 15 of input light 20. A crystal 25 of a given length L1 which is immersed in a magnetic field generated by a pair of magnets 30, which rotates the polarization state of the launched light 20 via the Faraday effect to an angle of 45°. An exit polarizer 35 is then aligned to a non-parallel polarization angle 40 so as to transmit this polarization state at the angle of 45° and pass light 20. Light 45, which has exited isolator 5 that may be reflected back into isolator 5, passes through the exit polarizer 35 experiences an additional rotation of polarization of 45° The polarization state of light 45 is then orthogonal to input polarizer angle 15 at the entrance plane to isolator 10. Optical isolation is thereby achieved in an optical system before and after isolator 5.
In many applications, the size of an optical isolator assembly is extremely important. The size has traditionally been limited by the Verdet constant of the isolator's Faraday material. In shorter wavelength applications of less than 1000 nm, the Verdet constant is low for optically desirable (e.g., low optical loss) materials such as Terbium Gallium Granite (TGG). Hence, a long optical path length is needed. Isolators with physical lengths of greater than 2 cm are common for these shorter wavelength applications. Accordingly, it would be most desirable to obtain an optical isolator for shorter wavelength applications with a more compact physical size.
Referring to FIG. 2, there is shown an optical element 5A which can rotate plane of polarization of polarized light 20A in a non-reciprocal manner, using Faraday-effect, which is called a Faraday rotator and is an essential building block for various optical devices such as isolator, circulator, and optical switch. A Faraday rotator 5A includes an optical element 25A and a magnet 30A surrounding the optical element 25A.
The magnet is magnetized and positioned in such a way that its magnetic field is aligned with an optical axis of the optical element. As a result, plane of polarization of polarized light 20A from a source such as a laser or an optical fiber traveling along the optical axis of the element will be rotated by a desired angle θ. This rotation may be clockwise or counterclockwise and its magnitude depends on Verdet constant V of optical element, magnetic field strength (B), and length of optical element (L). This rotation is expressed as:θ=VBL
During past several years various materials have been identified and made which can be used as an optical element for Faraday rotation. Among them Bismuth-Iron-Garnet composition (BIG) and TGG crystal are the most widely used materials. The value of Verdet constant, normally depends on the wavelength of the incident light and temperature, thus for a given wavelength and length of crystal a specific magnetic field B is needed to achieve a desired rotation angle θ. However, for BIG material the aforementioned linear equation is valid only up to certain level of magnetic flux strength, e.g., where B<350 Oe, and eventually the rotation angle will saturate and become constant by increasing the magnetic flux density B. This is an interesting feature for devices based on BIG material. As long as the magnetic field remains above the minimum saturation field the rotation angle will not change with any disturbance to B field due to temperature or proximity to other magnetic materials. Commercial single stage optical isolators with better than 40 dB isolation, less than 0.5 dB transmission loss, and few nano-meter band-width at 1550 nm are readily available. A major draw back for BIG materials is that their window of optical transparency is limited to above 1100 nm wavelength range and in visible and Near-IR wavelength (less than 1000 nm) BIG has large optical absorption loss and is not usable.
Magneto-optic crystals such as TGG crystal show very small optical absorption over large wavelength range including visible and NIR. However, magneto-optic crystals suffer from three fundamental problems. First, magneto-optic crystals have low Verdet constant compared to BIG. For example, the Verdet constant for TGG is two orders of magnitude less than that of BIG. Second, the rotation angle of magneto-optic crystals remains linearly proportional to B field for practical values of B field. This implies that a long crystal, e.g., on the order of tens of centimeters, along with a strong magnetic field (close to 1 Tesla) is needed in order to get about a 45° polarization rotation angle. In addition, to maintain constant rotation angle over life of the device, one has to make sure that the B field will not change by aging or disturbed by external perturbations such as temperature or proximity to other magnetic materials. Third, the use of multistage isolators are required to achieve isolation better than 30 dB. Such multistage isolators add to the cost and make the size of the isolator almost impractical for most applications.